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Body Size Variation among Adult Male Orangutans and its Implications for Sexual Dimorphism in Pongo spp.
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Abstract
Manuscript submitted to Integrative and Comparative Biology titled "Body Size Variation among Adult Male Orangutans and its Implications for Sexual Dimorphism in Pongo spp." currently in review in the journal Integrative and Comparative Biology
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Orangutans show a pronounced sexual dimorphism, with flanged males (i.e., males with fully grown secondary sexual characteristics) reaching twice the size of adult females. Furthermore, adult orangutans show sex-specific dispersal and activity patterns. This study investigates sex differences in adult foraging behavior and sheds light on how these differences develop in immatures. We analyzed 11 years of feeding data on ten adult female, seven flanged male, and 14 immature Bornean orangutans ( Pongo pygmaeus wurmbii ) at Tuanan in Central Kalimantan, Indonesia. We found that the diets of the adult females were significantly broader and required more processing steps before ingestion than the diets of flanged males. We also found evidence for a similar difference in overall diet repertoire sizes. For the immatures, we found that whereas females reached 100% of their mothers’ diet spectrum size by the age of weaning, males reached only around 80%. From the age of 4 years on (i.e., years before being weaned) females had significantly broader daily diets than males. We found no difference in daily or overall diet processing intensity of immature males and females but found preliminary evidence that immature males included fewer items of their mother’s diet in their own diets that were processing-intensive. Overall, our results suggest that by eating a broader variety and more complex to process food items, female orangutans go to greater lengths to achieve a balanced diet than males do. These behavioral differences are not just apparent in adult foraging behavior but also reflected in immature development from an early age on.
Significance Statement
In many species, males and females have different nutritional needs and are thus expected to show sex-specific foraging behavior. Sex differences in several aspects of foraging behavior have been found in various species, but it remains largely unclear when and how those develop during ontogeny, which is especially relevant for long-lived altricial species that learn foraging skills over many years. In our study, we analyzed a cross-sectional and longitudinal data set containing more than 750,000 feeding events of adult and immature Bornean orangutans ( Pongo pygmaeus wurmbii ). We found that adult females had significantly broader and more complex diets than males. We also found that these differences started to develop during infancy, suggesting that immature orangutans prepare for their sex-specific foraging niches long before those become physiologically relevant while they are still in constant association with their mothers and before being frequently exposed to other role models.
Orangutans are a species with a very pronounced sexual dimorphism, in that fully grown males are about twice the size of females, but adult, sexually mature males come in two distinct morphs. Unflanged males lack the secondary sexual characteristics (e.g., cheek flanges, throat sack, long call) of the flanged males, but are sexually active, fertile and known to sire offspring. In some males, full development may be delayed until they are over 30. This 'bimaturism' is hypothesized to have arisen as a result of sexual selection in which female choice, male-male competition and male coercion have all played important roles. The data reviewed here show that male-male competition is highly affected by the reproductive condition of the females and female preference for associating with particular males. Potentially reproductive female orangutans are widely dispersed in space and time. Consequently, dominant flanged adult males cannot easily exert permanent control over access to reproductive females.
Research on primate cognition has spurred interest in developmental influences on skill acquisition, especially complex foraging skills in great apes and specifically as they relate to species' life his-tory strategies. Survival skills are often mastered to functional levels near the onset of juvenility, for instance, this is when immatures are weaned and assume semi-independent lives. Several com-plications merit consideration: primate learning is lifelong; learners' situations change well beyond weaning; and individual tasks can vary such that they create rich problem spaces, mastering which entails acquiring and coordinating multifaceted skill sets. Accordingly, while skills may reach functional levels by weaning, they may be refined later. Juveniles, adolescents, and young adults, in particular, should generate such refinements given the developmental changes they experience in learning needs and opportunities, physical and cognitive abilities, and sociality. To assess acquisi-tion beyond weaning, this study tracked the acqui-sition of foraging skills for extracting heart from a tree palm (Borassodendron borneensis) in juvenile and adolescent rehabilitant orangutans. Findings represent 744 cases of palm heart extraction, 31 rehabilitants ranging in age from older infants to young adults, and two forests in Indonesian Borneo. Data were collected observationally over 9 y and include partial longitudinal data for 14 rehabilitants. Results highlight the importance of fine-grained assessments of both behavior and problem space in understanding developmen-tal influences on the acquisition of sophisticated foraging skills in great apes. Implications for the study of cetacean cognition are suggested.
Orangutans are usually solitary. However, occasionally aggregations are formed, especially in large fruiting fig trees. Individuals in these aggregations may experience scramble or contest competition for food. We investigated the type and strength of food competition in large figs among wild Sumatran orangutans. Adult males foraged more efficiently than adult females and subadult males did. The availability of ripe fruit is positively related to the number of orangutans visiting a fig tree and their foraging efficiency. The number of orangutans in a fig tree did not affect patch residence time and foraging behavior, though orangutans spent more time feeding when aggregation size increased in a fig tree. Dominance relationships could be measured in a number of dyads. Differences in dominance did not affect foraging behavior. The patch residence time of subordinate individuals was reduced on days that a dominant individual also visited the fig. In conclusion, orangutans seem to adjust aggregation size to the number of available ripe fruits in a fig tree in such a way that scramble competition was absent. Contest competition determined access to large fig trees.
Elucidating the somatic and maturational influences on the biomechanical properties of bone in children is crucial for a proper understanding of bone strength and quality in childhood and later life, and has significant potential for predicting adult fracture and osteoporosis risks. The ability of a long bone to resist bending and torsion is primarily a function of its cross-sectional geometric properties, and is negatively impacted by smaller external bone diameter. In pubescent girls, elevated levels of estrogen impede subperiosteal bone growth and increase endosteal bone deposition, resulting in bones averaging a smaller external and internal diameter relative to boys. In addition, given a well-documented secular trend for an earlier menarche, the age at which the rate of subperiosteal bone deposition decreases may also be younger in more recent cohorts of girls. In this study we examined the relationship between pubertal timing and subsequent bone strength in girls. Specifically, we investigated the effects of age at menarche on bone strength indicators (polar moment of inertia and section modulus) determined from cross-sectional geometry of the second metacarpal (MC2) using data derived from serial hand-wrist radiographs of female participants (N=223) in the Fels Longitudinal Study, with repeated measures of MC2 between the ages of 7 and 35 years. Using multivariate regression models, we evaluated the effects of age at menarche on associations between measures of bone strength in early adulthood and the same measures at a prepubertal age. Results indicate that later age at menarche is associated with stronger adult bone (in torsion and bending) when controlling for prepubertal bone strength (R(2) ranged between 0.54 and 0.70, p<0.001). Since cross-sectional properties of bone in childhood may have long lasting implications, they should be considered along with pubertal timing in assessing risk for future fracture and in clinical recommendations.
Although orang-utans live solitary lives most of the time, they have a complex social structure and are characterized by extreme sexual dimorphism. However, whereas some adult male orang-utans develop full secondary sexual characteristics, such as cheek flanges, others may stay in an "arrested" unflanged condition for up to 20 years after reaching sexual maturity. The result is a marked bimaturism among adult males. Flanged males allow females to overlap with their home range and often tolerate the presence of unflanged males. However, wherever possible flanged males actively prevent unflanged males from copulating with females. Two competing hypotheses, previously untested, have been advanced to explain male reproductive behavior and bimaturism in orang-utans: (1) the "range-guardian" hypothesis, which asserts that the flanged males are postreproductive and defend a range in which they tolerate sexually active, unflanged male relatives; and (2) the "female choice" hypothesis, which asserts that flanged males tolerate unflanged males in their range because they rely on female preference to favor flanged males. We investigated these hypotheses and a third hypothesis that the two male morphs represent co-existing alternative male reproductive strategies ("sitting, calling, and waiting" for flanged males versus "going, searching, and finding" for unflanged males). Fecal samples were collected from a well-studied population in Indonesia, and eight human microsatellites were analyzed for 30 individuals that have been behaviorally monitored for up to 27 years. By carrying out paternity analysis on 11 offspring born over 15 years, we found that unflanged males fathered about half (6) of the offspring. Relatedness between successful unflanged males and resident dominant males was significantly lower than 0.5, and for some unflanged/flanged male pairs, relatedness values were negative, indicating that unflanged males are not offspring of the flanged males. Copyright 2002.
Orangutans (Pongo spp.) are reported to have extremely slow life histories, including the longest average interbirth intervals of all mammals. Such slow life history can be viable only when unavoidable mortality is kept low. Thus, orangutans’ survivorship under natural conditions is expected to be extremely high. Previous estimates of orangutan life history were based on captive individuals living under very different circumstances or on small samples from wild populations. Here, we combine birth data from seven field sites, each with demographic data collection for at least 10 years (range 12-43 years) on wild orangutans to better document their life history. Using strict criteria for data inclusion, we calculated infant survival, interbirth intervals and female age at first reproduction, across species, subspecies and islands. We found an average closed interbirth interval of 7.6 years, as well as consistently very high pre-weaning survival for males and females. Female survival of 94% until age at first birth (at around age 15 years) was higher than reported for any other mammal species under natural conditions. Similarly, annual survival among parous females is very high, but longevity remains to be estimated. Current data suggest no major life history differences between Sumatran and Bornean orangutans. The high offspring survival is remarkable, noting that modern human populations seem to have reached the same level of survival only in the 20th century. The orangutans’ slow life history illustrates what can be achieved if a hominoid bauplan is exposed to low unavoidable mortality. Their high survival is likely due to their arboreal and non- gregarious lifestyle, and has allowed them to maintain viable populations, despite living in low- productivity habitats. However, their slow life history also implies that orangutans are highly vulnerable to a catastrophic population crash in the face of drastic habitat change.
Recent field data indicate that MacKinnon’s model of the orang-utan’s sexual and agonistic activity needs to be revised. In
this model, male reproductive activity is concentrated in an extended phase of subadulthood and in early adulthood. According
to this model, the role of older adult males is primarily that of range guardian, and in that role they would ensure that
the offspring they had generated earlier would have safe access to food resources. This study presents cases suggesting that
subadult males, even though sexually active, may have low reproductive success. In previous studies adult males were shown
to display less sexual initiative than subadult males. In this study an adult male was at times involved infrequent mating
activity in response to proceptive activity of females in the course of consortship. This adult male proved to be a successful
breeder, thus refuting the hypothesis of adult male sterility. The female is most likely to conceive through cooperative mating
in lengthy consortships with the dominant resident adult male. We hypothesize that the extended subadult phase represents
a submissive strategy, allowing subadult males to remain in the home range of adult males but with minimal reproductive success.
Among anthropoid primates there are interspecific differences in the degree of sexual dimorphism in both body size and canine
size. Within the suborder body size dimorphism and canine size dimorphism are positively correlated,r=0.76. This correlation suggests that the two dimorphisms are equally developed in some species, while in other species there
is a differential degree of sexual dimorphism.
An analysis of these results and their relation to social organization and other ecological variables reveals: (1) the degree
of canine size dimorphism is closely related to the amount of male intrasexual selection in a given mating system; and (2)
the degree of body size dimorphism is also related to male intrasexual selection, but may be modified (either enhanced or
diminished) by selection pressure from factors such as habitat, diet, foraging behavior, antipredator behavior, locomotory
behavior, and female preference.
For many years researchers have described some male orangutans as "subadult." These males are of adolescent to adult age and are reproductive, but have little to no secondary sexual trait development. Until now the only endocrine study of this arrest of secondary sexual trait development was performed by Kingsley (1982, 1988). She found that "subadult" or arrested males have lower testosterone levels than similar age developing adolescents or adult males. In this study, urine samples were collected over a two-year period from 23 captive male orangutans in order to more fully define male endocrine profiles. Three study males were juveniles, seven were arrested adolescents, six were developing adolescents, and seven were mature adults. Morning samples were analyzed by radioimmunoassay for levels of testicular steroids and gonadotropins and group hormone profiles were compared by analysis of variance. Results illustrate that arrested adolescent orangutans have significantly lower testosterone and dihydrotestosterone (DHT) levels than developing adolescents, but significantly higher levels than juveniles. Luteinizing hormone (LH) levels also differed between arrested and developing adolescents, with arrested males having lower levels. However, follicle stimulating hormone (FSH) levels were similar in both morphs of adolescent male. The overall hormone profiles for arrested and developing adolescent male orangutans suggest that arrested males lack levels of LH, testosterone, and DHT necessary for development of secondary sexual traits. However, they have sufficient testicular steroids, LH, and FSH to fully develop primary sexual function and fertility. These endocrine data help define alternative developmental pathways in male orangutans. The authors discuss the relationship between these developmental pathways and male orangutan reproductive strategies, and hypothesize about their prepubertal socioendocrine determination.
Mammals collected in eastern Sumatra by Dr. W. L. Abbott during 1903, 1906, and 1907, with descriptions of new species and subspecies
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